The corrosion of metals has widespread economic and environmental effects and also has a significant impact on public safety and health. The annual cost of corrosion to the US is estimated to be approximately 3% of GDP. A substantial part of that cost is due to atmospheric corrosion, and protection against atmospheric corrosion constitutes about 50% of all corrosion protection measures. Corrosion has led to bridge collapses, fatal airplane and train crashes, and the leakage and subsequent explosion of natural gas pipelines. The environmental health effects attributed to corrosion are also widespread. Structures such as storage tanks, pipelines, ships, railcars, and tanker trucks, which store and/or transport hazardous materials can be weakened and made unsafe by corrosion, and corrosion is also the leading cause of leaking chemical storage tanks.
Protective organic coatings (also known as paints) are one of the most cost-effective methods of preventing the corrosion of metals. These protective organic coatings are typically polymeric. In typical practice, the protective organic coating is applied over an inorganic conversion coating. The protective organic coating may comprise one or more layers of different organic coatings. The first layer is typically an epoxy that adheres well to the conversion coating and has excellent chemical and barrier properties. The epoxy coating is typically overlaid with a second coating, such as polyurethane, that is more resistant to weathering than the epoxy.
Conversion coatings are produced directly on the metal surfaces by treatment with a chemical agent [such as a soluble chromate or zinc phosphonate] to passivate or seal the surface. Conversion coatings are thin, new phases produced by the reaction of the metal and the chemical agent and are typically either metal phosphates or metal chromates. The conversion layers can enhance adhesion of the protective organic coating to the metal, provide an enhanced barrier to corrosion and can contain corrosion inhibitors.
The primary function of the protective organic coating is to prevent corrosion by physically blocking agents that cause corrosion (water, solubilizing organic or inorganic anions, certain oxidizing agents, etc.) from reaching the metal surface. However, this approach is defeated if the coating has a defect, if the coating becomes damaged or simply if water or other corrosive agents slowly penetrate through the intact coating. In order to deal with under-coat corrosion, or corrosion that occurs when the coating is damaged or defective, soluble or dispersible corrosion inhibitors are often added to the protective organic coatings.
A corrosion inhibitor for use in coatings is generally a soluble or dispersible material that is incorporated into the coating and can be transported by convection or diffusion to the site of corrosion where it slows down the corrosion reaction. The corrosion inhibitor must therefore be mobile and be able to migrate to the corrosion site, because the site is often a scratch or a gap in the coating that is not directly in contact with the coating itself.
Corrosion inhibitors can be divided into two broad categories, those that enhance the formation of a native protective oxide film through an oxidizing effect and those that inhibit corrosion by selectively adsorbing on the metal surface and creating a barrier that prevents access of the corrosive agent to the surface. In the former group are materials such as inorganic chromates, inorganic nitrates, molybdates and organic nitrates. The latter group includes materials such as carbonates, silicates and phosphates and organic molecules. containing heteroatoms such as nitrogen, sulfur, phosphorus and oxygen (e.g. materials such as anthranilic acid, thiols, organic phosphonates and organic carboxylates). Some of these materials also act as poisons for the cathodic oxygen reduction reaction that is linked to the anodic dissolution of the metal. Slowing down the cathodic reaction slows down the overall corrosion reaction.
Soluble chromates are widely used corrosion inhibitors due to their high effectiveness in arresting corrosion. Chromates are highly effective corrosion. inhibitors because they simultaneously provide several mechanisms to retard corrosion. (Frankel, G. S. and R. L. McCreery, “Inhibition of Al Alloy Corrosion by Chromates,” J. Electrochem. Soc., Interface, Winter, 34-38, 2001). Soluble chromates are oxidizing compounds that can also react with the corroding surfaces of aluminum and steel to provide an insoluble and a somewhat hydrophobic barrier. Chromates are also thought to inhibit corrosion by poisoning of the oxygen reduction reaction and inhibiting the initiation of corrosion. In current practice, high concentrations of chromates (sometimes up to 50% by weight) are added to the protective coating to provide a reservoir of the corrosion inhibitor so that a high level of corrosion protection can be maintained over several years, even in severe environments.
Unfortunately, soluble chromate corrosion inhibiting additives have adverse environmental effects, and there is a widely recognized need for non-chromate corrosion inhibitors. The toxic properties of chromates are well documented. The Public Health Service (ACGIH 1986/Ex. 1-3, p. 140) reports nasal irritation, evidence of liver enlargement and kidney dysfunction among chromate workers exposed to 0.06 to 0.07 mg Cr(VI)/m3. This report also cites excess lung cancer among chromate workers exposed to 0.01 to 0.15 mg/m3 soluble chromate and 0.1 to 0.58 mg/tm3 insoluble chromate. The use of chromate-inhibited spray-on coatings creates inhalable chromate tainted dusts. Removing chromate-inhibited coatings by chemical or mechanical means also generates a hazardous chromated waste that requires expensive disposal.
A number of chromate-like inorganics (e.g. molybdates, vanadates, and manganates) have been proposed as replacements for chromate conversion coatings and as additives for protective coatings (Cohen, S. M. “Replacements for Chromium Pretreatments on Aluminum”, Corrosion, 51(1), 71-78, 1995). Rare earth materials such cerium have also been evaluated as corrosion inhibitors (Mansfeld, F., V Wang and H. Shih “Development of “Stainless Aluminum”, J. Electrochem. Soc., 138(12), L74-L75, 1991). However, heavy metal chromate replacements may also be strictly regulated in the future.
Organic corrosion inhibitors are an alternative to the toxic heavy metal corrosion inhibitors currently used in. coatings. The inhibition of corrosion of a metal or an alloy by organic corrosion inhibitors can be achieved by many mechanisms, the effectiveness of which depends on many factors; including the nature of the metal, the oxidation-reduction potential of the environment, the temperature, and the concentration and strength of adsorption of the organic molecule to the metal surface. Organic corrosion inhibitors are generally low to moderate molecular weight molecules that primarily prevent corrosion by either reacting with the surface of the metal, its oxide, or its corrosion products to form a thin film. (Kuznetsov, Y. I., J. G. N. Thomas and A. D. Mercer. “Organic Inhibitors of Corrosion of Metals”, Plenum Pub Corp. 1996). Highly effective organic corrosion inhibitors generally interact with the metal via chemical adsorption. Chemical adsorption involves the formation of a coordinate bond between the metal surfaces and the organic corrosion inhibitor. The nature of the metal and the structure of the organic have a decisive effect on the strength of the bond and therefore the efficiency of the organic corrosion inhibitor. Organic corrosion inhibitors generally have donor atoms such as S, O and N that can donate electrons to the metal, thereby forming the coordinate bond. All other things being equal, higher electron density and larger polarizabilities usually lead to better corrosion protection, as known in the art. Because film formation is a chemical adsorption process, the temperature and the concentration of the inhibitors are also important factors in determining the effectiveness of the organic corrosion inhibitors. Corrosion inhibitors can be added directly to the protective organic coating, and using several different corrosion inhibitors can produce a synergistic effect For example, combinations of oleic acid and phenyl anthranilate have been reported to be significantly more effective than either of the inhibitors alone (Kuznetsov, Y. I., J. G. N. Thomas and A. D. Mercer. “Organic Inhibitors of Corrosion of Metals”, Plenum Pub Corp. (1996)).
Although there are numerous organic compounds that are excellent corrosion inhibitors in solution (V. S. Sastri, Corrosion Inhibitors: Principles and Applications, John Wiley and Sons, Chichseter, England 1998), these materials have yet to find widespread use in protective organic coatings. The primary technical reason for their lack of use is that the best organic corrosion inhibitors work because they contain functional groups (e.g. amines, amides, thiophenes, carboxylic acids, etc.) that form strong bonds to the metal surfaces. These same functional groups, can unfortunately, also react with the polymer resins used to produce the coating. The corrosion inhibitor is then locked into the polymer chain, thereby immobilizing it and preventing it from diffusing to the paint/metal interface where it is needed to block corrosion.
Even if the organic corrosion inhibitors are designed so that they would not be locked into the polymer structure (e.g. using latent reactive groups), when the corrosion-inhibited coatings are exposed to water (e.g. rain or aqueous detergent solutions used to clean the coatings), the inhibitors can be lost from the film by leaching, migration or extraction. The loss of inhibitor reduces the effectiveness and useful service lifetime of the coating. However, to work, the corrosion inhibitor must be able to diffuse through the coating to reach the corrosion site; especially if a hole or a scratch in the coating produced the corrosion site. Unfortunately, it is this mobility that allows the corrosion inhibitor to escape from the coating. Furthermore, if the coatings contain toxics (as do the currently used chromated epoxies), the toxics can be leached into the environment. In addition, adding a high concentration of a corrosion inhibitor to a coating can change the physical properties and chemical properties of a coating, often for the worse.
One way of solving these problems (e.g. immobilization of the inhibitor by reaction with the coating resins, loss to the environment and degradation of film properties) is by encapsulating the inhibitor molecule and using the encapsulant as an anti-corrosion pigment in a paint (J. D. Scantlebury and Dezhu Xiu, Journal of Corrosion Science and Engineering Abstract 22: A Sol-gel derived anti-corrosion pigment, http://www.umist.ac.uk/corrosion/JCSE/). Another approach is to ion-exchange the corrosion inhibitors onto a particle surface. Compositions that release corrosion inhibiting agents from particles include ion-exchange resins, ion-exchanged zeolites and carbon molecular sieves, ion-exchanged solid particles and water soluble glasses.
U.S. Pat. No. 3,899,624 discloses the use of organic ion-exchange resins incorporating corrosion inhibiting anions or cations and the release of said ions into a paint to arrest corrosion by ion exchange. The corrosion inhibiting ions include zinc and chromates. U.S. Pat. No. 4,738,720 discloses the use of a calcium ion-exchange zeolite composition and its use in a paint. H856, a statutory invention registration, discloses the use of calcium and barium exchanged Y-zeolites and their incorporation into a paint as corrosion inhibitors for steel panels.
U.S. Pat. Nos.6,383,271B1 discloses the use of fillers with hollow cellular structures such as diatomaceous earth, zeolite or carbon, wherein the hollow cells or pores are loaded with inhibitors or antioxidants as corrosion inhibitors for paints. Inhibitors disclosed include carbonic acids, amines, ketones, aldehydes, heterocyclic compounds, phosphates, benzoates, silicates, vanadates, tungstates, zirconates, borates, or molybdates.
U.S. Pat. Nos. 4,405,493, 4,419,137, 4,459,155, 4,474,607, 4,594,369, 4,643,769, 4,687,595, 4,749,550, 4,795,492, and 5,041,241 disclose compositions of alumina and silica inorganic particles whose surfaces are ion exchanged with corrosion inhibiting cations and anions including calcium, zinc, cobalt, lead, strontium, lithium, barium, magnesium, yttrium or cations of one or more metals of the lanthanide group, phosphates, chromates, benzoates or molybdates. The ion exchanged particle surfaces release their cations and ions via a subsequent ion-exchange thereby providing corrosion to metal substrates. U.S. Pat. Nos. 4,405,493, 4,419,137, 4,459,155, 4,474,607, 4,594,369, 4,687,595, 4,749,550, 4,795,492, 5,041,241 also provide for the incorporation of the ion-exchanged particles as corrosion inhibitors in paints.
In the above patents the corrosion inhibitors are ion-exchanged onto particle surfaces having ion-exchangeable groups. The corrosion inhibitors are released from the particle surfaces by a subsequent ion exchange with ions (e.g. chlorides, sulfates, sodium ions) transported into the coating via water penetrating through the coating. The present invention provides for chemically anchoring carboxylic acids to the surface of aluminum oxyhydroxide surfaces, as evidenced by quantum mechanical calculations based on Density Functional Theory and solid-state NMR studies. The chemically anchored corrosion inhibitors of the present invention are not released by ion-exchange, but they are released by chemical disruption of the carboxylate bond between the corrosion inhibitor and the aluminum oxyhydroxide surface.
U.S. Pat. Nos. 4,210,575, 4,428,774, 4,346,184, 4,518,429, and 4,561,896 disclose water soluble glass compositions, including as its major constituents phosphorous pentoxide and either zinc oxide or calcium oxide, which together form the glass forming oxide and glass modifying oxide respectively of the glass, together with a minor proportions of one or more oxides of an element or elements of Group IIA or Group IIIB of the periodic table, the compositions of the glass being such that, when the glass is contacted with water, phosphate ions and either zinc or calcium cations are leached into solution. The leached ions are disclosed as effective in the corrosion protection of iron or steel surfaces. These patents disclose that the glass material can be dispersed in a resin carrier, and thereby release corrosion inhibiting ions into the coating when the glass composition is contacted with water. In U.S. Pat. Nos. 4,210,575, 4,428,774, 4,346,184, 4,518,429, and 4,561,896, the corrosion inhibitors that comprise the water soluble glass are released when the glass dissolves upon contact with water.
U.S. Pat. No. 5,489,447 discloses the use of carrier bound ketocarboxylic acids as corrosion inhibitors. The ketocarboxylic acids are preferably bound to the surfaces of oxides, hydroxides, silicates or carbonates, where examples of these materials are alumina, magnesium oxide, aluminum hydroxide, magnesium hydroxide, kieselguhr, talc, aluminium silicate, calcium carbonate or iron oxide. These materials are incorporated into paints where they are disclosed to arrest corrosion.
In the present invention the material to which the corrosion inhibitors are chemically anchored are aluminum oxyhydroxides or inorganic particles that are fully or partially covered with aluminum oxyhydroxides, a class of materials not disclosed in U.S. Pat. No. 5,489,447. Furthermore, in the present invention the materials are designed to release the corrosion inhibitors under certain conditions, e.g. alkaline environments with pH greater than 9. The present invention provides for an improvement over the above disclosures. In all of the inventions described above (excepting U.S. Pat. No. 5,489,447), the materials are designed to release the corrosion inhibitors over time whether or not there is any corrosion occurring at the metal surface. The concentration of the organic inhibitor may therefore be reduced by leaching of the corrosion inhibitor from the coating before corrosion occurs. This reduces the effectiveness of the inhibitor and the effective service life of the coating.
The present invention describes methods and materials for providing the triggered release of organic corrosion inhibitors from particle carriers. The invention also provides for incorporating these corrosion inhibiting particles into protective organic coatings. In this invention, corrosion inhibitors are chemically anchored to a particle surface through a labile chemical bond that can be broken by interaction with hydroxide ions generated by corrosion of the metal surface. In the presence of oxygen most metals of practical interest corrode by anodic dissolution of the metal and cathodic reduction of oxygen, e.g.M→Mn++ne−O2+2H2O+4e−→4OH−The basic hydroxide ions generated by the corrosion process break the chemical bond between the corrosion inhibitor and the particle, and thereby release the previously anchored and immobilized corrosion inhibitor into the protective organic coating. In the present invention the corrosion inhibiting particles comprise one or more organic corrosion inhibitors that are covalently anchored to an aluminum oxyhydroxide surface through a carboxylic acid.
The triggered release of the anchored corrosion inhibitors from the aluminum oxyhydroxide surfaces of the present invention is observed when an aqueous dispersion of the carboxylate-anchored surface modified pseudoboehmite/boehmite particles is titrated. Above pH 6 to about pH 9 the solution viscosity increases, but no precipitation is observed. FTIR of the particles recovered from solution in this pH range shows that the organics are still anchored to the surfaces of the particles. However, above ˜pH 9, the particles precipitate out of solution. FTIR of the particles recovered from the latter experiment show no organic anchored to the surface and the quantitative recovery of the organics from the solution was achieved. Thus, the bond between the carboxylic acid and the pseudoboehmite/boehmite surfaces is unstable in basic conditions (i.e. above ˜pH 9).
There are several advantages to the present invention. The release of corrosion inhibitors is linked to and triggered by the corrosion process. Since the release of the organic corrosion inhibitors occurs only when triggered by the corrosion processes, this minimizes the amount of corrosion inhibitor that can be leached out of the coating. Secondly, the invention allows multiple organic corrosion inhibitors to be incorporated simultaneously into a protective polymer coating at concentrations sufficient to inhibit corrosion without degrading the physical properties and performance of the coating (either by anchoring different types of inhibitors to a single particle or by using two or more types of particles each with a single type of inhibitor attached). This means organic corrosion inhibitors that are active over a wide range of pH conditions (corrosion can also be occurring at additional sites where the electrolyte conditions are neutral or acidic) are available in the coating for arresting corrosion. The ability to chemically anchor multiple types of releasable corrosion inhibitors to the particle carriers is also important since numerous studies have shown that mixed organic corrosion inhibiting agents can have a synergistic effect.
The materials described in the present invention are of class of materials known as alumoxanes. U.S. Pat. No. 5,593,781 discloses preparation of alumoxanes by surface modification of pseudoboehmite powders of nanometer size particles with small molecular weight organic compounds in a one-step process by dispersing the ceramic powder in water or an organic solvent and adding the low molecular weight organic compound. Apblett et al. [Mat. Res. Symp. Proc. Vol. 249 1992] also disclose the formation of carboxy substituted particles from the reaction of pseudoboehmite and carboxylic acids in a one-step process. Landry et al. [J. Mater. Chem. 1995, 5(2), 331-341] describe the reaction of [Al(O)(OH)]n with carboxylic acids to form [Al(O)x(OH)y(O2CR)z]n where R=C1-C13 and 2x+y+z=3 using a one-step reaction. U.S. Pat. No. 6,369,183 discloses thermoset polymer networks formed from surface modified carboxylate-anchored amine, hydroxyl, acrylic and vinyl modified aluminum oxyhydroxide particles. However, the above patents do not disclose the use of carboxylate surface-modified aluminum oxyhydroxide particles or inorganic (non-aluminum oxyhydroxide) particles whose surfaces are coated with an aluminum oxyhydroxide and then carboxylate surface modified that provide for the triggered release of corrosion inhibitors